Follow the step-by-step examples to learn how to use five major host-based intrusion detection tools: TCP Wrappers, xinetd, Tripwire, Swatch, and PortSentry. You will also be introduced to simple tools that can be used to create powerful baseline and auditing methods for your systems.

This chapter is from the book

Exercise 1: TCP Wrappers

Description

For a connection request to be serviced, a process needs to be active and
listening on its well-known port (ports less than 1024). This means that
processor capacity and memory must be allocated for each service that a server
is going to host. In the early days of Unix, this was a very expensive waste of
valuable, scarce resources.

To make more effective use of these resources, inetd was developed to act as
a super listener or meta dameon for connection requests. When a request for a
new session is received, inetd activates the appropriate service. For example,
if a server is to host FTP, Telnet, and SMTP (email), there should be an active
service listening to ports 21, 23, and 25, respectively. With inetd running,
only one process listens to all three ports. If a request for a new Telnet
session is received, then inetd starts the Telnet service on port 25 for the
session.

While this is an effective means to minimize demands on the processor, it
does nothing for security. Most significantly, it leaves security to be handled
by each service. This, in turn, leads to inconsistent and often weak security
implementations for the network services that inetd assists. For example, there
are no security mechanisms involved in establishing a Telnet session. It leaves
security to the login process, which enforces security on the basis of username
and password. This makes the system vulnerable to brute-force, password attacks
and can also leak potentially useful information through the login banner.

TCP Wrappers addresses this shortcoming of inetd by inserting itself into the
middle of the process that establishes connections. It does this by changing the
inetd configuration file, inetd.conf, so instead of inetd activating the
requested service, it actually activates the TCP Wrappers daemon,
in.tcpd. If TCP Wrappers determines that the connection can be allowed,
then it starts the service process.

It is the goal of TCP Wrappers to log the origination of incoming connection
attempts and to provide more granular control of the process. This approach also
makes it easier to configure a server, and it is a more consistent approach for
securing the server.

While TCP Wrappers greatly improves the security of a system, it is not
without its own weaknesses. It can provide a false sense of security because
some of its checks are inadequate and user authentication is not reliable. But,
its biggest drawback is that it relies too strongly on IP addresses, which can
be spoofed undetectably.

Newer versions of Linux have replaced TCP Wrappers with their own version
called xinetd.

Objective

Red Hat Linux 7.2 uses xinetd instead of TCP Wrappers. Consequently, we will
not complete a hands-on session with TCP Wrappers. However, if you have access
to a system that uses TCP Wrappers, you can follow this exercise to configure
it. Since this exercise will use TCP Wrappers, you will not be able to perform
the steps if your system uses xinetd.

The primary objective of this exercise is to examine the configuration files
used by TCP Wrappers to perform its functions.

Requirements

Permission
If you are going to review the configuration of a system that utilizes TCP
Wrappers, you should obtain authorization from the legal owner and/or your
management team prior to conducting this exercise.

Hardware
None

Software
None

Challenge Procedure

The following are the steps you need to perform for this exercise:

Examine an entry from inetd.conf for a system that doesn't
use TCP Wrappers.

Modify the entry to utilize TCP Wrappers.

Configure a system to selectively allow connections.

Configure a system to selectively deny connections.

Challenge Procedure Step-by-Step

The following are the steps you are going to perform for this exercise:

First, you'll examine an entry from inetd.conf for a system
that doesn't use TCP Wrappers. The inetd daemon knows how to manage
the services it controls based on the entries in /etc/inetd.conf.
This file is a space-delimited file that consists of a series of single-line
entries. Each entry corresponds to a service that is invoked by inetd. Each
standard service that inetd can start has a well-known port assigned to
it. Each service must have a valid entry in /etc/services. In the
case of an internal service, its name must correspond to the official name
of the service, which is the first field of the service entry in /etc/services.
The following figure shows a sample entry from inetd.conf.

Press Shift+R, and then type tcp. This should cause vi to
overwrite the entry with TCP. Press the Esc key to exit overtype
mode. Press ZZ to save the change and exit vi.

Next, you'll configure the system to selectively allow connections.
After inetd has been configured, it is necessary to configure TCP Wrappers
to tell it what connections to grant or deny. Two files are used for this
purpose/etc/hosts.allow and /etc/hosts.deny. As their
names imply, hosts.allow defines what connections to allow, and hosts.deny
defines what connections to deny.

The easiest approach to configuring TCP Wrappers is to determine the default
network policy and then configure the files accordingly. It is very important
to note that the hosts.allow file is processed before hosts.deny.
This means that you should be careful not to make any entries in hosts.allow
that may accidentally grant access to a system that is denied access in
hosts.deny. In other words, if the default network policy is to deny
all network connections except for a limited few, then you should configure
hosts.deny to deny everyone and hosts.allow to allow only
a few connections. But, if your default policy is to allow everyone except
for a limited few, you should not configure hosts.allow to allow everyone
and hosts.deny to deny a few. In this case, since there is a match
in hosts.allow, hosts.deny will never be processed.

In this example, we will set up a system with a deny-all-except policy.
Only systems from the sans.org domain will be granted access.

Configure /etc/hosts.deny. To do this, log in as root and change
the directory to /etc:

cd /etc

Start vi to edit hosts.deny:

vi hosts.deny

Delete any other entries in the file. Press the colon (:)
key. Your cursor should go to the bottom-left position on the screen. Type
the following command:

Next, you'll configure a system to selectively deny connections. In
this example, you set up a system with an allow-all-except policy. Since
the system has been experiencing a lot of attacks from blackhats.com, systems
from that domain will be denied access. All others will be granted access.

Configure hosts.deny. To do this, log in as root, and then change
the directory to /etc:

cd /etc

Start vi to edit hosts.deny:

vi hosts.deny

Delete any other entries in the file. Press the colon key. Your
cursor should go to the bottom-left position on the screen. Type this command:

Summary

TCP Wrappers provides a significant security enhancement to Unix-based
systems. With it, you gain greater control over who can have access to your
system's resources. But, don't let TCP Wrappers give you a false sense
of security. It is also vulnerable to spoofing attacks.

In addition to increased access control, TCP Wrappers logs all incoming
connection requests. Even if the login is successful, this information can still
be useful for indicating if a system has been penetrated. For example, a review
of the logs could indicate connections at unusual times. This may be an
indication that the user's account has been compromised and is being used
by another person. It may also indicate that a user is performing activities he
is not normally authorized to perform.